Primary outcomes, secondary outcomes, and their relevance to how many papers are published from a study: A primer for authors, reviewers, and editors

In research, outcomes are often categorized as primary and secondary. The primary outcome is the most important one; it determines whether the study is considered 'successful' or not. Secondary outcomes are chosen because they provide supporting evidence for the results of the primary outcome or additional information about the subject being studied. For reasons that are explained in this paper, secondary outcomes should be cautiously interpreted. There are varying practices regarding publishing secondary outcomes. Some authors publish these separately, while others include them in the main publication. In some contexts, the former can lead to concerns about the quality and relevance of the data being published. In this article, we discuss primary and secondary outcomes, the importance and interpretation of secondary outcomes, and considerations for publishing multiple outcomes in separate papers. We also discuss the special case of secondary analyses and post hoc analyses and provide guidance on good publishing practices. Throughout the article, we use relevant examples to make these concepts easier to understand. While the article is primarily aimed at early career researchers, it offers insights that may be helpful to researchers, reviewers, and editors across all levels of expertise.